1. Field of the Invention
The present invention relates to a pyroelectric thin film infrared sensor and a method for fabricating the same.
2. Description of the Prior Art
Pyroelectric thin film infrared sensors are generally used for realizing more convenient sensing systems because of their advantages of high sensitivity at long wavelength and operability at room temperature without use of any cooling system.
Referring to FIG. 1, there is illustrated an example of a conventional pyroelectric thin film infrared sensor. As shown in FIG. 1, the pyroelectric thin film infrared sensor includes a substrate 1 and a pyroelectric thin film 2 centrally disposed on the upper surface of substrate 1. Over the pyroelectric thin film 2, an Ni--Cr electrode 3 is disposed. A polyimide layer 5 is disposed over the entire exposed upper surface of the resulting structure including the exposed upper surface of the Ni--Cr electrode 3 and the exposed upper surface of the substrate 1. The polyimide layer 5 has a contact hole 4 through which the Ni--Cr electrode 3 is partially exposed at its upper surface. Over one side portion (the right portion in FIG. 1 ) of the upper surface of polyimide layer 5, a read-out electrode 6 made of an aluminum layer is disposed. The read-out electrode 6 is electrically connected to the Ni--Cr electrode 3 through the contact hole 4. Another polyimide layer 7 is disposed over the read-out electrode 6 except for the right end of the read-out electrode 6. The pyroelectric thin film infrared sensor further includes another Ni--Cr electrode 9 disposed beneath the substrate 1. The Ni--Cr electrode 9 is electrically connected to the lower surface of pyroelectric thin film 2 exposed through a hole 8 formed in the substrate 1 by use of an etching process. The Ni--Cr electrode 9 is disposed on the right portion of the surface of hole 8 and the right portion of the lower surface portion of substrate I not etched upon forming the hole 8.
Now, a method for fabricating the conventional pyroelectric thin film infrared sensor with the above-mentioned structure will be described, in conjunction with FIGS. 2A to 2E.
In accordance with this method, first, a substrate having (100)-oriented plane is prepared as the substrate 1, as shown in FIG. 2A. The substrate 1 may be an MgO single crystal substrate.
Over the substrate 1, a PbTiO.sub.3 -based pyroelectric thin film is deposited to a thickness of 3 .mu.m using a radio frequency (RF)-magnetron sputtering process. The pyroelectric thin film has a perovskite structure and C-axis orientation. Thereafter, the pyroelectric thin film is subjected to a well-known photolithography process so that it remains only at its central portion. The remaining central portion of pyroelectric thin film constitutes the pyroelectric thin film 2 centrally disposed over the substrate 1.
Subsequently, a Ni--Cr thin film is deposited to a thickness of 0.4 .mu.m over the resulting structure including the pyroelectric thin film 2 and the substrate 1 by use of a vacuum evaporation process, as shown in FIG. 2B. The Ni--Cr thin film is then selectively etched using the well-known photoetch process so that it is etched except for its portion disposed over the pyroelectric thin film 2, thereby forming the Ni--Cr electrode 3.
Over the entire exposed upper surface of the resulting structure, a polyimide layer is then deposited as the polyimide layer 5. This polyimide layer 5 is selectively etched using the well-known photoetch process so as to form a contact hole in the polyimide layer 5. The contact hole constitute the contact hole 4 for centrally exposing the upper surface of Ni--Cr electrode 3 therethrough.
As shown in FIG. 2C, an aluminum layer is then deposited over the entire exposed upper surface of the resulting structure including the upper surface of Ni--Cr electrode 3 and the exposed upper surface portion of substrate 1 by use of a sputtering process or an evaporation process. Subsequently, the aluminum layer is selectively etched using the well-known photoetch process so that it remains at its portion disposed over one side portion, namely, the right portion of the polyimide layer 5. The remaining portion of aluminum layer constitutes the read-out electrode 6.
Thereafter, formation of the polyimide layer 7 is carried out using the well-known photolithography process. The polyimide layer 7 is disposed over the read-out electrode 6 except for one side end, namely, the right end of the read-out electrode 6. The polyimide layer 7 is also disposed over a portion of the polyimide layer 5 adjacent to the left end of the read-out electrode 6.
As shown in FIG. 2D, the substrate 1 is subjected to a well-known anisotropic etch process at its portion disposed beneath the pyroelectric thin film 2, thereby forming the hole 8. In this case, the substrate 1 is anisotropically etched by use of an H.sub.3 PO.sub.4 solution until the lower surface of pyroelectric thin film 2 is completely exposed. After completion of the etching, the pyroelectric thin film 2 is supported in position only by the polyimide layer 5.
A Ni--Cr layer for absorption of infrared rays is then deposited over the entire exposed lower surface of the resulting structure including the side wall surface of hole 8, the lower surface portion of substrate 1 not etched, and the exposed lower surface of pyroelectric thin film 2, by use of the sputtering process or the evaporation process, as shown in FIG. 2E. Subsequently, the Ni--Cr layer is then selectively etched using the well-known photoetch process so that it remains only at the right side wall surface portion of hole 8 and the right portion of the lower surface portion of substrate 1. The remaining portion of the Ni--Cr layer constitutes the Ni--Cr electrode 9.
Finally, a subsequent packaging step is carried out. At the packaging step, the Ni--Cr electrode 9 is bonded 1o a corresponding one of leads of a lead frame not shown by a conductive paste. The exposed portion of lead-out electrode 6 is wire-boned to a gate of a junction field effect transistor (JFET) not shown by an Au wire.
In accordance with the above-mentioned conventional method, however, the lower surface of pyroelectric thin film 2 may be damaged by the etching solution upon anisotropically etching the lower surface of MgO single crystal substrate 1.
Furthermore, the conventional thin film infrared sensor also has a problem that the packaging step becomes complex because the Ni--Cr electrode for absorption of infrared rays can not be wire-bonded to the lead of lead frame. This is because the Ni--Cr electrode is disposed on the lower surface of substrate.